1. Field of the Invention
The present invention relates to a drive circuit of a solid-state image pickup device, a method of driving a solid-state image pickup device and an image pickup system of a solid-state image pickup device.
2. Description of the Related Art
An image pickup apparatus such as a digital camera includes a CMOS image sensor being an image pickup device of XY address type as an image pickup device. A CMOS image sensor is advantageously smaller than a CCD in occurrence of smear to an ignorable extent.
However, as a rolling shutter, a CMOS image sensor of XY address type accumulates charges at timings different for each line and hardly finishes an accumulation operation concurrently for all the pixels. An intent to control time for accumulating charges of a CMOS image sensor and control exposure results in displacement of the accumulation period by nearly one frame between the first line and the last line of scanning lines. Therefore, the CMOS image sensor deems not to be suitable for picking up an image of a moving object to be picked up as a still image. Therefore, a mechanical shutter for controlling exposure time of a CMOS image sensor is frequently used.
Here, a reset operation of a CMOS image sensor in each line for starting accumulation of charges will be executed prior to timing of a read operation at a signal level of accumulation charges in each line by time required for charge accumulation time. That resetting can be made different from the read at signal level of accumulation charges in operation speed. By utilization thereof, Japanese Patent Application Laid-Open No. H11-041523 has disclosed that a CMOS image sensor includes a reset operation to be carried out on a line basis at a speed to go along the running of a mechanical shutter to control exposure.
Japanese Patent Application Laid-Open No. H11-041523 includes charge accumulation to be started subjected to a reset operation to be carried out on a line basis at a speed to go along the running of a mechanical shutter and then shield light with a mechanical shutter and thereafter to carry out an operation of reading signal levels of accumulation charges on a line basis. Adjustment of the period for the reset operation and the running operation of the mechanical shutter enables exposure control at the time of image pickup.
In Japanese Patent Application Laid-Open No. H11-041523, in order to synchronize the reset operation to be carried out on a line basis with the running of the mechanical shutter that does not necessarily operate at a constant speed, a clock for vertical scanning among scanning clocks generated by a pulse generator undergoes frequency modulation with a vertical drive modulation unit and is supplied to an image pickup device.
There is a shutter mechanism including a reset operation of a CMOS image sensor as a front blade and a mechanical shutter as a rear blade. In order for the shutter mechanism to obtain a good image without exposure unevenness, it is necessary to accurately adjust reset timing of each line of a CMOS image sensor in conformity with running characteristics of a mechanical shutter being a rear blade to make exposure time of each line of a sensor constant.
On the other hand, a mechanical shutter blade is generally driven with a spring and is retained at the starting position of running mostly by attraction of an electromagnet. Therefore, due to a plurality of factors such as posture difference, temperature and moisture of an image pickup device, drive voltage of an electromagnet retaining a mechanical shutter and individual difference in the mechanical shutters, the running characteristics of a mechanical shutter is not always constant.
Therefore, it is necessary for a drive circuit of an image pickup device to generate accurate reset timing for each line of the image pickup device in conformity with the state of an image pickup apparatus at the time of picking up an image.
Japanese Patent Application Laid-Open No. 2006-033338 has disclosed the following method. With an amount of charge accumulated between one of scanning for causing each region of an image pickup device to start accumulation of charge and scanning for reading the charge accumulated in each region, and running of a mechanical shutter shielding an acceptance surface of the image pickup device, the running characteristics of a mechanical shutter is calculated. Then, accurate reset timing for each line of the image pickup element being the front blade is generated.
In addition, Japanese Patent Application Laid-Open No. 2002-051270 has disclosed an example of a timing generator of generating a great number of timing pulses used for driving a solid image pickup device.
In one of Japanese Patent Application Laid-Open No. H11-041523 and Japanese Patent Application Laid-Open No. 2006-033338, a pulse for vertical scanning among scanning pulses generated by a pulse generator undergoes frequency modulation in a vertical drive modulation section and is supplied to an image pickup device. Bringing the characteristics of vertical scanning speed and the running characteristics of a mechanical shutter into matching, that configuration can generate pulse for vertical scanning the interval of which changes non-linearly. However, it will become difficult to maintain relative relation between the pulse for vertical scanning and the pulse other than the pulse for vertical scanning. In order to discharge a photodiode in a reset operation of the photodiode of a solid-state image pickup device, the photodiode needs to be in a reset state for not less than a constant period. However, a pulse for one of starting and concluding resetting the photodiode will be required other than the pulse for vertical scanning. Therefore, such a configuration hardly maintaining relative relation between the pulse for vertical scanning and the pulse other than the pulse for vertical scanning is inappropriate. In addition, a modulation unit of controlling the vertical scanning speed is required to increase costs.
In addition, according to a memory built-in timing generator disclosed in Japanese Patent Application Laid-Open No. 2002-051270, in order to generate a pulse for vertical scanning the interval of which changes non-linearly, all the changing points of the pulse must be stored in the memory. For example, the case of a solid-state image pickup device with six million pixels including 3000×2000 pixels generating reset timing to be a front blade with a memory built-in timing generator will be described. In that case, since all the reset timing for respective 2000 lines of the image pickup device must be stored in memory, the circuit size of the memory built-in timing generator will increase to increase costs. In addition, in the case where posture difference, temperature and moisture of an image pickup device, drive voltage of an electromagnet retaining a mechanical shutter and the like fluctuate, the running characteristics of the mechanical shutter change at each occurrence. Therefore, communication with a CPU and the like will become necessary for setting pulse for vertical scanning into the timing generator, undesirably increasing communication time in order to store all reset timings for each line of the image pickup device.
An object of the present invention is to provide a drive circuit of a solid-state image pickup device, a method of driving solid-state image pickup device and an image pickup system of solid-state image pickup device capable of generating a pulse for vertical scanning the interval of which changes non-linearly, a pulse other than the pulse for vertical scanning and the like without increasing a circuit size and communication time for setting an electronic shutter.